Blaustein et al. (1997) used a field experiment to investigate whether increased UV-B radiation was a cause of amphibian deformities (see the photo at the beginning of this chapter). They measured long-toed salamanders either exposed to or shielded from natural UV-B radiation. It was not possible to carry out all replicates simultaneously, so the researchers carried them out over several days. They made sure that both treatments were included on each day. In their analysis, they grouped replicates together that were carried out on the same day.

a. By grouping experiments carried out on the same day, what experimental procedure were they using?

b. What is the main reason for adopting this procedure in an experimental study?